1. Field of the Invention
The present invention relates to improved color imaging systems and methods of making and using the same and, in particular embodiments, to such systems and methods for improved color image printing wherein the status of a picture element is determined based, in part, on error factors which are dependent on the status of neighboring picture elements.
2. Description of Related Art
One well known process of forming an image (such as a graphic design, alphanumeric character, or other indicia formed by a printer, cathode ray tube, LED or other electronic display device) is to divide the area in which the image is to be formed into a plurality of discrete picture elements ("pels"). Each pel is then selectively enhanced, such that the combined plurality of pels forms the desired image.
In many printing environments, the printing system's ability to "selectively enhance" a picture element is "bi-level", in that it is limited to either forming a dot (a black, bright or color spot) or not forming a dot at the picture element location. In such hi-level systems, selected areas (composed of multiple pels) of the image may be made to appear in various shades (e.g., shades of grey, bright or color) by enhancing various numbers of interspaced pels located within the area.
Spacial grey scale techniques for controlling a printer or other imaging apparatus have been used in such bi-level systems to improve the visual appearance of the shaded areas of the printed image. One example of such spacial grey scale techniques is described in the article titled "An Adaptive Algorithm for Spatial Grey Scale" by Robert Floyd and Louis Steinberg, published in SID Digest ("Floyd et al").
Floyd et al. describe an error diffusion system, wherein the processing of one pel in an image is dependent upon the processing of other pels in the image, to provide a more continuous tone image with a bi-level device. An algorithm is used to determine the manner in which the error (the difference between the desired level of darkness and the actual state of the picture element) is diffused among neighboring pels.
According to Floyd et al.'s system, the "actual" enhancement or brightness of a pel is either 0 (dark) or 1 (bright). However, the "desired" brightness at that pel's location in the picture may be anywhere between or including 0 and 1. Accordingly, each pel introduces an error (the difference between the "actual" state of the pel and the "desired" brightness level of the pel) into the picture. For example, if the "desired" brightness level for a pel is 1/4, but the pel's "actual" state is dark (0), an error of -1/4 is introduced into the picture. On the other hand, if the "desired" brightness for the pel is 1/4 and the "actual" state of the pel is bright (1), an error of +3/4 is introduced into the picture. Floyd et al. employ the matrix or "kernel" below to determine the distribution of (diffuse) the error introduced by each pel.
______________________________________ * 7 3 5 1 ______________________________________
The "*" represents the pel being processed and the numbers "7", "3", "5" and "1" represent weighting factors and locations for neighboring pels that have not yet been processed. The error introduced by the "*" pel is distributed as 7/16th's, 3/16th's, 5/16th's and 1/16th's to pels 7, 3, 5 and 1, respectively. The "Jarvis kernel," described by Jarvis, et al. shown below is an enhancement of the Floyd et al.
______________________________________ * 7 5 3 5 7 5 3 1 3 5 3 1 ______________________________________
However, such matrix systems previously tended to generate defects near the peripheral edges of the image. Moreover, the image quality and continuity of tone in the image were often not as optimal as desired. As a result, the images formed by such systems tended to be blurry and lacking in detail.
Another technique for "grey scaling" is referred to as "screening," wherein the level of enhancement (darkness, brightness or color) of a pel is determined by the number of adjacently disposed small dots which are formed at the pel location. Thus, a lightly enhanced pel may be formed of one or two adjacently disposed small dots, while a highly enhanced pel may be formed of, for example, nine adjacently disposed small dots. According to further grey scaling techniques, the level of enhancement of a pel is controlled by adjusting the size of the dot formed at the pel location. However, these processes require a printing system which is capable of forming a selectable and variable number of adjacently disposed small dots or a selectable and variable size dot for each pel location (i.e., the printing system must be other than a bi-level system).